This application is based upon and claims the benefit of priority from Chinese Patent Application No. 01107066.8, filed Jan. 21, 2001, the entire contents of this application are incorporated herein by reference.
The present invention relates to a static correction method for exploration seismic data using the first arrivals of seismic waves recorded by receiver gathers on the earth surface. More particularly, the present invention relates to a shorter spatial wavelength static correction method for smoothing the time values of the first arrivals such as refracted seismic waves which are picked up from seismic gather records after the field static correction, to obtain shorter spatial wavelength static correction.
Seismic exploration method is the most widely used and effective geophysical technique in oil and gas exploration for locating the drilling sites. The seismic waves generated by artificial sources travel into the earth and return to the surface after reflection from interfaces between formations having acoustic impedance contrasts. The reflection and/or refraction of the seismic waves generated on the earth surface by respective shot points, which are arranged and moved in a predetermined regular manner, are recorded by receivers for detecting the generated seismic waves, which are laid along the ground at distances from the shot points in a predetermined manner. Variations in the reflection times from place to place indicate structural features in the strata underground. The seismic structure figures those come from seismic data processing and interpretation are the most important information to locate the drilling sites. Therefore, the detected arrival times and waveforms of the reflection generated by the interfaces between the geology formations on the seismic waves are processed and analyzed, and the status and location of the geology formations can be acquired and determined.
Due to the differences between various geology formations underground in composition, density, and uniformity of distribution etc., propagation velocities of the seismic waves generated by the shot points are correspondingly different therein, and the respective arrival times and waveforms of the reflection and/or refraction waves, which are generated by the interfaces between the formations having velocity and acoustic impedance contrasts and return to the surface of the earth, detected by the receivers are different from each other. Among these data, the first arrivals such as refracted waves, which are earliest detected and most significant and valuable for static corrections during processing, are the returned seismic waves after refracted by the lower boundary of the weathered low velocity layer (LVL) covering the land surface, such as desert and loess plateau, and mountainous area.
Usually the land surface is covered with a weathered layer of low velocity. The topography is never flat, the lower boundary of LVL is never planar. Variation in thickness and velocity of the upper layer can cause travel time delay or priority for waves to the surface. The reflections are diverged from the normal rolexe2x80x94the hyperbola relationship between the arrival times and offsets(receiverxe2x80x94shot distances). It can cause a dramatic deterioration in the quality of seismic data. The reflection energy can not be focused in horizontal stack and the images of reflectors in seismic section become ambiguous. So static corrections for eliminating the divergence of wave arrivals are very important and become a key step of seismic data processing in exploration regions with complex LVL.
Investigations show that the particular geology formations of certain LVL have same influence on the divergence of the arrival time of the reflection waves from deeper strata as that for the first arrival waves refracted from the LVLs. By performing seismic static correction to the time values of the first arrivals detected by the respective receivers iteratively and in various manners, to eliminate the divergence of the arrival time of the first arrival waves refracted in the LVLs, as a result, the divergence of the reflected waves from the formations of deeper strata can be also eliminated, so that the formation of the deeper strata can be focused in the seismic data and the image thereof can be more clear.
So far, the most powerful techniques for receiver and shot point static corrections are refraction statics. They are used after field static correction and before residual static correction. The waves refracted by the interfaces between the LVL and consolidated rocks below are transmitted back to surface before the direct arrival and reflection arrivals, they are known as the headwaves and become conventional first arrivals in seismic records.
The conventional method of static corrections for receiver and shot point includes steps of: performing coarse field static correction to the original exploration seismic data recorded by each receiver using height and surface measurement; performing refraction static correction to the first arrivals, i.e., obtaining the LVL thickness and velocity by means of inversion of the structure of the LVL, then performing static correction by calculating the time difference; and performing residual static correction as well as other processes to the seismic data. However, because of the complexity of LVL model design and the non-uniqueness of inversion, the accuracy of the conventional refraction static usually is rather low for shorter spatial wavelength statics and they can not be satisfied in complex LVL regions in practice.
On the other hand, there are two modes in current seismic exploration: 2-D seismic survey and 3-D seismic survey. In 2-D seismic survey usually the shot points and receivers are laid along an exploration line, and the shot points are moved along the exploration line after each shot to get the exploration data, until the detection of one line is completed. After acquiring the data of one exploration line, data for other exploration lines can be acquired in the same manner. For 3-D seismic survey, the receivers of a shot are laid in several lines (for example, q) covered a region, and the shot points are also distributed in several lines (for example, p). Usually shot lines and receiver lines are in the same direction but not in same places, so that a two dimensional pxc3x97q exploration array is formed. During the data acquiring process of 3-D seismic survey, after each of the shot points in an array is activated once respectively to record the exploration waves, the whole exploration array is moved forward to activate all the shot points once again, until the detection of the whole area is completed.
Furthermore, the static correction for 3-D seismic exploration, especially the shorter spatial wavelength static correction using the first arrivals is much more complex than that for 2-D seismic exploration, and there is no effective method to convert 3-D seismic exploration data into 2-D seismic exploration data in static processing. Therefore, the shorter wave static correction for exploration seismic data using the first arrivals of refraction waves are mainly used in 2-D seismic data processing. It is difficult and thus rarely to apply shorter spatial wavelength static correction to 3-D seismic data processing.
In view of the above, an object of the present invention is to provide a novel shorter spatial wave static correction method for exploration seismic data using the first arrivals, which is simple in operation, and has better correction results. With the method of this invention, the static correction value can be obtained directly, and the curves of first arrivals can be smoothed, so that the reflection waves can be better focused and have more clear images.
Another object of the invention is to provide a shorter spatial wave static correction method, which is applicable to 2-D ad 3-D exploration seismic data both, especially applicable to the static correction of 3-D seismic data using first arrivals, and can also obtain satisfied correction results.
To achieve the objects of the invention, the present invention provides a shorter spatial wavelength static correction method for exploration seismic data using first arrivals, comprising the steps of:
a)performing fitting on the first arrival times picked up from the seismic records of receiver gathers after field static correction, to obtain a fitted curve of the first arrival time values with respect to distances between respective receiver points and shot points;
b) obtaining the time difference xcex94i,j between the first arrival time tij detected by each of the receiver points j with respect to each of the shot points i and the respective time values on the fitted curve; and forming the original matrix xcex94 having mxc3x97n elements xcex94i,j as follows:   Δ  =      [                                        Δ                          1              ,              1                                                            Δ                          1              ,              2                                                …                                      Δ                          1              ,              j                                                …                                      Δ                          1              ,              m                                                                        Δ                          2              ,              1                                                            Δ                                          2                ,                2                            ⁢                              xe2x80x83                                                              …                                      Δ                          2              ,              j                                                …                                      Δ                          2              ,              m                                                            ⋮                                      xe2x80x83                                                xe2x80x83                                                xe2x80x83                                                xe2x80x83                                                xe2x80x83                                                            Δ                          i              ,              1                                                            Δ                          i              ,              2                                                …                                      Δ                          i              ,              j                                                …                                      Δ                          i              ,              m                                                            ⋮                                      xe2x80x83                                                xe2x80x83                                                xe2x80x83                                                xe2x80x83                                                xe2x80x83                                                            Δ                          n              ,              1                                                            Δ                          n              ,              2                                                …                                      Δ                          n              ,              j                                                …                                      Δ                          n              ,              m                                            ]  
where i is the number of shot point and j is the number of receiver point, the size of matrix xcex94 is nxc3x97m, where n is the total number of shot points and m is the total number of receiver points of the line, i.e., xcex94i,j indicates the difference between observed first arrival and the time value of the fitted curve for the jth receiver point of the ith shot point.
c) taking average of the values for the elements of each row in the matrix xcex94, and obtaining the difference xcex94xe2x80x2i,j between the value xcex94i,j and the row average as follows:
where m1 is the total number of elements with value which is not zero in the ith       Δ          i      ,      j        xe2x80x2    =            Δ              i        ,        j              -                            ∑                      i            =            1                                m            1                          ⁢                  Δ                      i            ,            j                                      m        1            
row of the matrix xcex94;
d) substituting xcex94xe2x80x2i,j for xcex94i,j in the matrix xcex94, and form the matrix xcex94xe2x80x2:             Δ      xe2x80x2        =          [                                                                  Δ                xe2x80x2                                            1                ,                1                                                                        Δ                              1                ,                2                            xe2x80x2                                            …                                              Δ                              1                ,                j                            xe2x80x2                                            …                                              Δ                              1                ,                m                            xe2x80x2                                                                                          Δ                xe2x80x2                                            2                ,                                  1                  xe2x80x2                                                                                        Δ                                                2                  ,                  2                                ⁢                                  xe2x80x83                                            xe2x80x2                                            …                                              Δ                              2                ,                j                            xe2x80x2                                            …                                              Δ                              2                ,                m                            xe2x80x2                                                            ⋮                                              xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                                          Δ                              i                ,                1                            xe2x80x2                                                          Δ                              i                ,                2                            xe2x80x2                                            …                                              Δ                              i                ,                j                            xe2x80x2                                            …                                              Δ                              i                ,                m                            xe2x80x2                                                            ⋮                                              xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                                          Δ                              n                ,                1                            xe2x80x2                                                          Δ                              n                ,                2                            xe2x80x2                                            …                                              Δ                              n                ,                j                            xe2x80x2                                            …                                              Δ                              n                ,                m                            xe2x80x2                                          ]        ;
e) taking average of the values for the elements of each column in the matrix xcex94xe2x80x2, and calculates the differences:       Δ          i      ,      j        xe2x80x2xe2x80x2    =            Δ              i        ,        j            xe2x80x2        -                            ∑                      j            =            1                                n            1                          ⁢                  Δ                      i            ,            j                    xe2x80x2                            n        1            
where n1 is the total number of elements with value which is not zero in the jth column of the matrix xcex94xe2x80x2;
f) substituting xcex94xe2x80x3i,j for xcex94xe2x80x2i,j in the matrix xcex94xe2x80x2 and forming the matrix xcex94xe2x80x3:             Δ      xe2x80x3        =          [                                                                  Δ                xe2x80x3                                            1                ,                1                                                                        Δ                              1                ,                2                            xe2x80x3                                            …                                              Δ                              1                ,                j                            xe2x80x3                                            …                                              Δ                              1                ,                m                            xe2x80x3                                                                                          Δ                xe2x80x3                                            2                ,                1                                                                        Δ                                                2                  ,                  2                                ⁢                                  xe2x80x83                                            xe2x80x3                                            …                                              Δ                              2                ,                j                            xe2x80x3                                            …                                              Δ                              2                ,                m                            xe2x80x3                                                            ⋮                                              xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                                          Δ                              i                ,                1                            xe2x80x3                                                          Δ                              i                ,                2                            xe2x80x3                                            …                                              Δ                              i                ,                j                            xe2x80x3                                            …                                              Δ                              i                ,                m                            xe2x80x3                                                            ⋮                                              xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                          xe2x80x83                                                                          Δ                              n                ,                1                            xe2x80x3                                                          Δ                              n                ,                2                            xe2x80x3                                            …                                              Δ                              n                ,                j                            xe2x80x3                                            …                                              Δ                              n                ,                m                            xe2x80x3                                          ]        ;
g) repeat the above steps c) to f) to perform iterations, until the value of |xcex94i,j| become less than a given small value, which means the iteration process is converged;
h) finding sum of the averages of the elements in ith row for all iterations as the static correction value at the ith shot point, and finding sum of the averages of the elements in the jth column for all iterations as the static correction value at the jth receiver point; and
i) perform static correction to the exploration seismic data using said static correction values for the respective ith shot points and jth receiver points, to obtain corrected seismic data.
For 3-D seismic surveys, the shorter spatial wavelength static correction method according to the invention further comprises a step of using formula of rij={square root over (xcex94xij2+xcex94yij2)}, before the step a), to transform 3-D seismic data into 2-D data in the case of 3-D seismic surveys, where xcex94xij is the abscissa difference between ith shot point and jth receiver, xcex94yij is the ordinate difference between ith shot point and jth receiver, and rij is the distance difference between ith shot point and jth receiver, so that the 3-D seismic data processing is as simple as that of 2-D data.